The Coronary Drug Project (CDP) was undertaken to evaluate several lipid-lowering drugs in the long-term treatment of coronary heart disease and to study the natural history of this disease. The CDP is a multi-clinic study consisting of 53 Clinical Centers, a Coordinating Center, a Central Laboratory, an ECG Reading Center, and a Drug Procurement and Distribution Center. 8,341 patients were enrolled in the study during recruitment from March 1966 through October 1969. Only males between the ages of 30 and 64 having a history of at least one ECG-documented myocardial infarction and belonging to NYHA functional class I or II were eligible for enrollment. Each patient was randomly assigned to one of six treatment groups: mixed conjugated equine estrogens (two dosage levels), clofibrate, dextrothyroxine, nicotinic acid, and placebo. All patients are to be followed for a minimum of 5 years on their assigned CDP medication plus a final 4 months off their CDP medication unless definitive results allow termination before 5 years. The terminal 4-month period will end in February 1975. Information from this study is being used to evaluate the CDP drugs with respect to mortality, cardiovascular morbidity and serum lipids as well as side effects, biochemical effects and electrocardiographic changes. Placebo group data on the demographic, clinical electrocardiographic, biochemical, pharmacologic and radiographic variables observed at baseline as well as other aspects of the natural history of coronary heart disease will be analyzed for prognostic significance. Patients in whom active medication was discontinued because of a protocol change have been allocated to treatment with either aspirin or placebo in a double blind study. The purpose of this study is to evaluate the effect of aspirin-induced inhibition of platelet aggregation on coronary atherosclerosis.